Driving control of a reciprocating CPR apparatus

ABSTRACT

A method of controlling the amount of compressed gas used for driving a reciprocating apparatus for cardio-pulmonary resuscitation (CPR) comprising a valve means for controlling the provision of driving gas comprises operation of the valve means during the compression phase to stop provision of driving gas, which operation is separated in time from the venting of the driving gas from the apparatus at the end of the compression phase. Also disclosed are; a CPR apparatus operated by the method; a method of compression depth sensing.

RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.12/523,082, filed Aug. 13, 2009 and currently pending, which is anational stage entry of International patent application no.PCT/SE2008/000022, filed Jan. 14, 2008, which claims the benefit ofSwedish Application No. 0700094-6, filed Jan. 18, 2007.

FIELD OF THE INVENTION

The present invention relates to a method of sensing the chestcompression depth in a reciprocating apparatus for cardio-pulmonaryresuscitation (CPR) driven by a compressed gas, a method of controllingthe amount of compressed gas used for driving a reciprocating apparatusfor cardiopulmonary resuscitation (CPR), and a correspondingly operatedreciprocating CPR apparatus.

BACKGROUND OF THE INVENTION

In cardio-pulmonary resuscitation (CPR) repeated compressions areadministered by hand or by apparatus to the chest of the person beingresuscitated to maintain circulation and oxygenation of blood.Concomitant with the compressions electrical shocks are provided to thepatient to make the heart beat again. Gas-driven reciprocating CPRapparatus have been known in the art and used in practice for a longtime; see, for instance, U.S. Pat. No. 3,209,747 (Guentner) and U.S.Pat. No. 3,277,887 (Thomas). Providing compressions of correct depth isan important factor for success of the method.

In the following “Compression Depth” signifies the maximum sternaldeflection during a compression/decompression cycle. An appropriateCompression Depth for adult persons corresponds to a sternal deflectionof 20%; the compression depth for a chest with an anterior-posteriordiameter of 25 cm thus is 5 cm. In contrast “compression depth” in thefollowing refers to a sternal deflection during acompression/decompression cycle smaller than the maximum deflection orto sternal deflection in general.

Shallow compressions may be insufficient to restore circulation andoxygenation while compressions that are too deep may damage the ribs andthe soft tissues of the chest. There is thus an optimal CompressionDepth or a narrow range of optimal Compression Depths. Administration ofcompressions of optimal Compression Depth may be controlled byadministering compressions of a given force. Alternatively, a desiredCompression Depth may be set by an operator; it may be optionallychanged during resuscitation. Alternatively, the Compression Depth in aCPR apparatus can be set by limiting the stroke of the piston in theapparatus to the average optimal Compression Depth for an adult person.A given compression force results in a compression to a CompressionDepth at which the compression force is balanced by the resistive forceof the chest tissues. Since even adult persons differ in their chestanatomy a given compression force may result in compression of varyingdepth in a group of persons. Therefore the direct determination ofCompression Depth during cardiopulmonary resuscitation and its use forcontrol of the apparatus by which the compressions are administered isdesirable.

The determination of compression depths during cardiopulmonaryresuscitation is known in the art. A corresponding device and a methodbased on accelerometer measurements disclosed in U.S. Pat. No.7,118,542. An accelerometer-based compression monitor is placed on thepatient's sternum, the arm of a rescuer administrating manual heartcompressions or on a compression-administrating part of an automatic CPRdevice. The chest is then compressed. The accelerometer signal isintegrated and fed to a processor, which calculates the compressiondepth from the signal by use of complex algorithms. The accelerometer iselectrically connected to the processor.

In the administration of repeated compressions in cardiopulmonaryresuscitation the use of apparatus based on a reciprocating pistonprovided with a chest compression pad and mounted in a cylinder isknown. The piston is driven by a compressed gas. An apparatus of thiskind dedicated for use by off-hospital medical emergency teams isdisclosed in US 2003/0181834. The Compression Depth administered withsuch an apparatus is limited by physical means comprised by theapparatus and set from start to from about 40 mm to about 50 mm for anadult person.

A problem with CPR apparatus driven by a compressed gas is limited gassupply, since the apparatus may be used for an hour or more to providecompressions to the patient during transport to a hospital by, forinstance, an ambulance.

OBJECTS OF THE INVENTION

It is an object of the invention to provide a method of determining theCompression Depth in cardiopulmonary resuscitation and, optionally, acompression depth, in particular as administered by a reciprocatingapparatus for cardio-pulmonary resuscitation driven by a compressed gas,in particular by a compressed breathing gas, in which the CompressionDepth is not limited by physical means comprised by the apparatus.

It is another object of the invention to provide such determination overone or multiple compression cycles.

It is further object of the invention to provide a corresponding means.

Still another object of the invention to provide a method of optimizingthe use of compressed breathing gas for driving the apparatus based onthe determination of the Compression Depth and/or a compression depth.

An additional object of the invention is to provide a reciprocatingapparatus for cardio-pulmonary resuscitation driven by a compressed gas,provided with said means and control means for optimizing the use ofdriving gas.

Still further objects of the invention will become apparent from thestudy of the following summary of the invention, the description ofpreferred embodiments thereof illustrated in a drawing, and the appendedclaims.

SUMMARY OF THE INVENTION

According to the present invention is disclosed a method of sensing theCompression Depth and/or a compression depth in a reciprocatingapparatus for cardio-pulmonary resuscitation (CPR) driven by acompressed gas and comprising a reciprocating part and anon-reciprocating part, the method comprising arranging a sensor mountedon the non-reciprocating part capable of sensing a signal emanating fromthe reciprocating part at a selected position thereof; transmitting thesignal to a microprocessor unit that records its arrival time.

It is preferred for the non-reciprocating part to comprise a cylindricalhousing having a top wall, a bottom wall and a side wall, and for thereciprocating part to comprise a piston disposed in the housing so as toallow displacement thereof by a compressed gas adduced to an uppercompartment in the housing defined by the top wall, a portion of theside wall, and the piston. Additionally, displacement in the oppositedirection may be effected by adducing a compressed gas to a lowercompartment in the housing defined by the bottom wall, a portion of theside wall, and the piston. When administering CPR compressions to arecumbent patient the housing is disposed above the patient's chest. Acompression pad fixed at one end of a plunger or piston rod extendingthrough a bore in the bottom wall, the other end of which is fixed tothe piston, is placed on the sternal region of the chest. In thisapplication indications of direction and position, such as “upper”,“upwards”, “top” and “lower”, “bottom”, “downwards”, are governed bythis disposition of the CPR apparatus in respect of the patient.

The cylinder walls and the piston are preferably of a diamagneticmaterial, in particular an organic polymer material, for instancepolystyrene or polyester, in particular polyamide, possibly reinforcedwith organic or inorganic fibre.

While contactless sensing is preferred, sensing by contact is comprisedby the method of the invention. In contactless sensing it is preferredto sweep one or several magnetic or radiation sensors mounted at thenon-reciprocating part with a magnetic field or radiation, respectively,emanating from a corresponding field or radiation source mounted at ordeflected or reflected by the reciprocating part, thereby giving rise toan electric signal in a swept sensor. The radiation source is oneemitting radiation in the UV, visible or IR range of the spectrum.Alternatively the radiation source is an ultrasound source, and theradiation detector is an ultrasound detector.

It is preferred to keep the Compression Depth of the apparatus constantby physical means, and for a contact sensor to be activated at themoment when the reciprocating part reaches the Compression Depth.

It is preferred for the one or several magnetic sensors to be disposedon the outer face of the side wall, in particular in a line or arrayextending in an axial direction. In this application an axial directionis one extending in parallel with the axis of the cylindrical housing.The magnetic field source is a permanent ring or rod magnet mounted onor in the piston. In such case the piston should be of a diamagneticmaterial, such as glass or carbon fibre reinforced polyamide.Particularly suited materials for the ring or rod magnet are Al—Ni,neodymium and samarium cobalt. Preferred magnetic sensors compriseHall-effect elements such as unipolar, bipolar, and omnipolarHall-effect digital switches.

It is preferred for the radiation source to be disposed at the innerface of the bottom, its radiation being directed at the bottom face ofthe piston, which is diffusely or mirroring reflective or comprises adiffusely or mirroring reflective material capable of reflecting ormirroring, respectively, of more than 20%, preferably more than 50%,even more preferred more than 80% or the incident radiation. It ispreferred for the one or several radiation sensors to be disposed at theinner face of the bottom. If there are two or more radiation sensorsthey are preferably disposed in a line or an array, such as a line orarray extending in a radial direction. In this application “radialdirection” is a direction extending radially from the centre of thepiston. The detection of the position of the piston may be based ontotal reflected radiation detected by the one or several radiationsensors or by detection of reflected radiation by two or more radiationsensors so disposed that each sensor receives a maximum of reflectedradiation from a position of the piston that is different from that fromwhich an adjacent sensor receives a maximum of reflected radiation. Itis preferred for the radiation source to be a source of visible, UV orIR light. A preferred source of radiation is a light-emitting diode(LED). Preferred optical sensors are sensors capable of detecting theradiation emitted from the radiation source, in particular sensorscapable of detecting radiation reflected by the reflective material,such as photodiodes and phototransistors.

The contactless method of the invention and the corresponding means thusallow to monitor the position of the compression pad over time, such asover one or several compression cycles. In an reciprocating CPRapparatus with a pre-set compression depth by use of a physicalCompression Depth limiter it is possible to monitor the moment at which,or whether, the Compression Depth is reached by an electrical contactmounted at or in proximity of the limiter, which is actuated by thepiston so as to close or open an electrical circuit, thereby producingan electrical recordable signal.

According to an important aspect of the invention, which is explainedbelow in detail, monitoring the Compression Depth and/or a compressiondepth can be used to optimize, that is, to minimize the consumption ofdriving gas in a reciprocating CPR apparatus of the aforementioned kind.This allows to substantially reduce the consumption of driving gas byCPR apparatus.

The initial resistance of the chest of adult persons to a compression of50 mm is in the order of about 300 N to 400 N. To provide desired rapidcompressions at a rate of from about 60 min.sup.-1 to 120 min.sup.-1 andmore, a gas-driven reciprocating CPR apparatus has to be driven with apressure that is substantially higher than the pressure required to justovercome this resistance. This pressure is, of course, also dependent onthe area of the piston in the CPR apparatus. From a standpoint ofdriving gas supply economy, a suitable driving gas pressure is fromabout 2.5 to 4 bar, preferably about 3 bar. A suitable piston area for adriving gas pressure of 3 bar is about 20 cm.sup.2. This driving gaspressure is however only needed during a short period from the start ofproviding chest compressions to the patient, such as for a period of oneor two minutes. During this period the resistance of the chest decreasesby about 50 percent. In consequence, a lower pressure is required toadminister compressions of same depth during the following steady-statedperiod in which the resistance of the chest does not change.

In a compression cycle of the steady-state period the provision ofdriving gas to the CPR apparatus is only required during a portion ofthe compression phase, in particular over a period extending from theonset of the compression phase to shortly before the compression depthis reached, such as from 10 to 50 milliseconds, more preferred from 20to 40 milliseconds, most preferred about 30 milliseconds before theCompression Depth is reached. Thereby the consumption of driving gas canbe substantially reduced, such as by 40 percent and even up to 50percent. By definition the onset of the compression phase is determinedby the opening of the valve controlling the supply of gas to thecylinder. The cylinder starts its downward movement shortly thereafter.The end of the compression phase and the start of the decompressionphase is determined by the opening of a valve for venting the useddriving gas from the upper compartment. At the same time, the piston canbe returned to its starting position by adducing compressed driving gasto the lower compartment. Thereby the compression pad, which abuts thebreast of the patient and carries a circumferential soft sealing rim oris provided with an adhesive, brings the patient's breast back to itsoriginal position. In the absence of such breast returning means thepassive return by the chest's resilience is much slower and not itsoriginal position.

According to the present invention is also disclosed a method ofcontrolling the amount of compressed gas, such as compressed air, usedfor driving a CPR apparatus of the aforementioned kind over areciprocating cycle, comprising a method of determining the compressionDepth and/or a compression depth, including determining the moment atwhich the Compression Depth and/or a compression depth is reached; thatmoment is preferably determined by sensor means mounted at anon-reciprocating part of the apparatus sensing radiation reflected froma reciprocating part of the apparatus or a magnetic field moving with areciprocating part of the apparatus.

The method of controlling the amount of compressed gas used for drivinga CPR apparatus is based on the insight that the supply of compressedgas to the compartment of the cylinder in which the gas expands anddrives the piston against the resistance of the patient's chest tissuesis a dynamic process. In this process equilibrium between the gaspressure in the reservoir, in which partially decompressed gas from thegas cylinder is stored at an about constant pressure, and saidcompartment is not established. To safeguard the working of theapparatus within specifications such as compression rate and waveformthe pressure of the gas fed to the compartment must be substantiallyhigher than the pressure that is needed to displace the piston to theCompression Depth in a situation of equilibrium. According to theinvention the amount of gas used required for compression to theCompression Depth can be substantially reduced by stopping the supply ofgas to the compartment at the moment when the compression depth isreached or, preferably, shortly before that moment.

In the method of controlling the amount of compressed gas the moment atwhich the Compression Depth and/or a compression depth is reached, whichmoment may have been determined in (a) preceding reciprocating cycle(s),is used for positional control of valve means through which thecompressed gas is adduced to the upper compartment in the housing. Atthe moment at which a selected position, in particular the CompressionDepth, is reached during a reciprocating cycle, that is, when the pistonincluding a compression pad mounted to the piston is in its extremedownward position, or at a prior point in time during that cycle, themicroprocessor means operates the valve means, for instance a solenoidvalve control unit comprising one or several solenoid valves, so as tostop the flow of compressed gas to the upper compartment of the housing.It is preferred for the prior point in time to be from 0 to 50milliseconds prior, in particular from 0 to 30 milliseconds prior, mostpreferred about 10 to 15 milliseconds prior to the moment at which thecompression depth is reached. In addition and independently thereof itis preferred for the valve means to stop providing driving gas to theupper compartment after at point in time when the pressure of the gas inthe upper compartment is from 30 to 70 percent of the pressure of thedriving gas, more preferred from 40 to 60 percent, most preferred about50 percent. A preferred driving gas pressure is from 2.5 to 4 bar, morepreferred about 3 bar. It is also preferred for the portion of thecompression phase during which the valve means allow driving gas toenter the upper compartment to extend from the start of the compressionphase until a point in time when from 25 percent to 40 percent of thelength of the compression phase has passed.

In particular, the method of controlling the amount of compressed gasused for driving a reciprocating apparatus for cardiopulmonaryresuscitation (CPR) comprising a valve means for controlling theprovision of driving gas to a reciprocating part of the apparatus andfor venting of the used driving gas from the apparatus, comprises: (a)operating the valve means at the start of a compression/decompressioncycle to provide driving gas; (b) operating the valve means during thecompression phase to stop provision of driving gas; (c) operating thevalve means at the end of the compression phase to vent the driving gasfrom the apparatus. Step (b) is preferably initiated at a point in timewhen the pressure of the provided driving gas is from 30 percent to 70percent of the pressure of the driving gas, more preferred from 40percent to 60 percent of the pressure of the driving gas, most preferredabout 50 percent. It is also preferred for step (b) to be initiated at apoint in time which is at from 25 percent to 40 percent of the length ofthe compression phase, in particular at a point in time before themoment at which a compression pad of the reciprocating part has reachedthe Compression Depth.

According to the present invention is additionally provided a method ofcontrolling the amount of compressed gas, such as compressed breathinggas, used for driving a CPR apparatus of the aforementioned kind but inwhich the piston stroke, that is, the Compression Depth, is mechanicallycontrolled. Mechanical control of the Compression Depth is obtained bymechanically stopping the piston at a desired length of stroke by, forinstance, the bottom of the cylinder or a circumferential flangearranged on the inner face of the cylinder between the piston and thebottom. The method of control comprises determining the moment at whichthe Compression Depth is reached during a reciprocating cycle, whichmoment may have been determined in (a) preceding reciprocating cycle(s).Alternatively and/or additionally, the method of control comprisesdetermining the moment at which a compression depth is reached during areciprocating cycle, which moment may have been determined in (a)preceding reciprocating cycle(s). The means for determining the momentat which the Compression Depth and/or a compression depth is reachedinclude those described above but can also, for instance, comprise asimple mechanically operated electric switch that is switched by impactof, for instance, the piston at the moment or close to the moment atwhich the piston is stopped by the stopping means. The method is usedfor positional control of valve means through which the compressed gaspasses into the upper compartment in the housing. At the moment when theCompression Depth is reached (that is, when the piston is in its extremedownward position) or slightly prior to that moment, the microprocessormeans operates the valve means, for instance a solenoid valve controlunit comprising one or several solenoid valves, so as to stop the flowof compressed gas to the upper compartment of the housing. “Slightlyprior to that moment” signifies from 0 to 50 milliseconds, in particularfrom 0 to 30 milliseconds, most preferred about 10 to 15 millisecondsprior to the moment at which the piston reaches the compression depth.The stroke length of the CPR apparatus in which the compression depth ismechanically controlled may be fixed or can be set prior to or evenduring CPR.

According to the present invention is disclosed a CPR apparatuscomprising a housing and a reciprocating piston mounted in the housingdriven by a pressurized gas, in particular a pressurized breathing gassuch as air. The apparatus further comprises a means for sensing thecompression depth, in particular a means for contactless sensing. Acompression pad is mounted to the piston via a plunger and is placed onthe chest of a patient above the sternum. The reciprocating movement ofthe piston is so transferred to the pad, and it is by the pad that thepatient's chest is compressed to the Compression Depth. The frequency ofcompression can be varied but is typically in the order of 60 to 120cycles per minute.

It is preferred for the sensing means to comprise a source of radiationor of a magnetic field and one or several radiation or magnetic fieldsensors, respectively. It is preferred for the source of radiation andfor the optical sensor(s) to be mounted at a non-reciprocating part ofthe apparatus and for the sensing means to comprise a reflective areadisposed on a top or bottom face of the piston. It is preferred for thesource of magnetic field to be mounted at a reciprocating part of theapparatus, in particular the piston, and for the field detection sourceto be mounted at a non-reciprocating part of the apparatus, inparticular an inner face of the bottom wall or the top wall. Theradiation or magnetic sensor(s) are electrically connected to amicroprocessor unit capable of comparing the sensor signal with a givensignal and to store sensor signal data over one or more cycles.

According to a particularly advantageous aspect of the invention the CPRapparatus comprises a valve means, in particular a solenoid valve means,controlled by means of compression depth and time data stored in themicroprocessor memory related to the time at which the piston reachedthe Compression Depth during a reciprocating cycle, in particular anearlier reciprocating cycle.

The control of driving gas according to the method of the invention doesnot comprise the optional provision of driving gas to embodiments of theCPR apparatus of the invention at the end of the compression phase topush the piston back to its starting position.

SHORT DESCRIPTION OF THE DRAWINGS

The invention will now be explained by reference to preferred but notlimiting embodiments thereof illustrated in a rough drawing, in which

FIG. 1 is a sectional view of a first embodiment of the apparatus of theinvention disposed on the chest of a patient shown in a transversesection at the level of the eight thoracic vertebra (T8) and viewed in acranial direction;

FIG. 1a is a detail view of the embodiment of FIG. 1, in the same viewand enlarged;

FIG. 1b is a section A-A (FIG. 1) through a modification of theapparatus of FIG. 1;

FIG. 1c is a detail view of another modification of the apparatus ofFIG. 1 showing solenoid valve control unit with a pair of solenoidvalves;

FIGS. 2a-2h show the embodiment of FIG. 1 and in the same view, inconsecutive states of chest compression by reciprocating displacement ofits piston and compressing pad;

FIG. 3 is a block scheme of a solenoid valve control program;

FIGS. 4a-4d show another embodiment of the apparatus of the invention,in the same view as in FIG. 1 and, in FIGS. 4c and 4d , partiallyenlarged;

FIG. 5 shows a further embodiment of the apparatus of the invention, inthe same view as in FIG. 1;

FIGS. 6a-6c are graphs illustrating the effect of driving gas valveopening times on gas consumption in reaching and maintaining a desiredcompression depth against a given resilient force in CPR modelexperiments;

FIG. 7 is a rough sketch of the compression testing apparatus used inthe experiments illustrated in FIGS. 6a -6 c.

DESCRIPTION OF PREFERRED EMBODIMENTS Example 1

The embodiment of the apparatus 1 of the invention shown in FIGS. 1-2 hcomprises a cylinder housing of a diamagnetic material having a sidewall 2, a bottom 3 and a top wall 4. A piston 5 with a circumferentialsealing 9 is mounted in the housing and defines an upper compartment Aand a lower compartment B. A plunger 6 extends downwards from the centreof the piston 5, passing through a central bore in the bottom 3 of thehousing. At its free end the plunger 6 carries a chest compression pad 7provided with a flexible circumferential lip 8. The piston 5/plunger6/compression pad 7 is mounted displaceably in the cylinder housing. Aneodymium magnet ring 14 is mounted at the lower face of the piston 5with its south pole S facing the side wall 2. An array of unipolarHall-Effect digital switches (“unipolar Hall switches”) 15, 16, 17, 18,19 is mounted at the outer wall of the cylinder 1 in an axial direction.The unipolar Hall switches 15, 16, 17, 18, 19 are characterized by theirmagnetic operating threshold. If the Hall cell of a switch 15, 16, 17,18, 19 is exposed to the magnetic field of the south pole exceeding theoperating threshold, the output transistor is switched on. If the fielddrops below the switching threshold, the transistor is switched off. Onepole of each of the unipolar Hall switches 15, 16, 17, 18, 19 isgrounded at 23 whereas the other is fed with 3 V DC by lines 15′, 16′,17′, 18′, 19′, respectively, connected to a microprocessor unit 13. InFIG. 1a the field lines 24 of the magnet's 14 south pole S are shown inrespect of unipolar Hall switches 18, 19 to illustrate how the latterare influenced during a displacement of the plunger 6. The effect (Halleffect) by which the switches 15, 16, 17, 18, 19 are closed by theinfluence of the field of the magnet 14 allows to monitor the passage ofthe plunger by the microprocessor unit 13 (FIG. 3). During passage ofthe magnet 14 the circuit of the respective switch 15, 16, 17, 18 or 19is closed, the current passing a closed switch being recorded by themicroprocessor unit 13. After passage of the magnetic field therespective switch is again opened except for if the plunger stops in aposition in which the magnetic field does cover it after stop. Thisallows the microprocessor unit 13 to keep track of the movement of thepiston 5/plunger 6/pad 7 assembly and, in particular, its position atthe end of its downward or, less important, upward movement, and tocontrol the provision of compressed breathing gas to compartment A bythe solenoid valve based on that position.

For reasons of simplicity number of unipolar Hall switches in theembodiment of FIGS. 1 to 2 h is confined to five. An embodiment thatallows to obtain fine tuning of positional control can comprise a highernumber of unipolar Hall switches and/or have the switches disposed inthe region of the compression depth level where the determination ofposition of piston 5/plunger 6/compression pad 7 is most important.Other Hall-effect switches like bipolar and omnipolar Hall-effectswitches may be used for sending the field of magnet 14.

In a modification of the embodiment of FIG. 1 the ring magnet 14 isexchanged for a rod magnet 6′ (FIG. 1b ). For reasons of balance acounter weight 27 is mounted diametrically opposite to the magnet 6′ atthe lower face of the piston 5′. The use of a rod magnet 6′ requires thearrangement of a means preventing rotation of the piston 5′. In FIG. 1bthe rotation preventing means comprises two diametrically oppositeaxially extending flanges 28 protruding from inner face of the cylinderside wall 2′ and co-operating with diametrically opposite axiallyextending slits in the side wall of piston 5′. Also shown is aHall-effect switch 18′ mounted at the outer face of the side wall 3′opposite to the south pole of magnet 14′.

Returning to the embodiment of FIGS. 1 and 2 a-2 h the upper compartmentA of the housing is defined by the top face of the piston 5, a firstportion of the side wall 2 of the housing, and the top wall 4 of thehousing, whereas its lower compartment B is defined by the bottom faceof the piston 5, the bottom face of the magnet 14, a second portion ofthe side wall 2 and the bottom wall 3. An opening 22 in the bottom wall3 allows air to enter into compartment B or to be expelled from itdepending on the direction of displacement of the piston 5.

A tube 10 for providing compressed breathing gas from a gas supply suchas a gas cylinder or other container of compressed breathing gas (notshown) is mounted at and communicates with an opening 25 in the top wall4. Near the opening 25 a venting tube 21 branches off from tube 10. Tube21 can be put in communication with a breathing mask (not shown) borneby the patient under cardiopulmonary resuscitation. A three-way solenoidvalve 11 controlled by a solenoid control unit 12 is mounted in thelumen of tube 10 at the branching of the venting tube 21. In a firstposition P1 the solenoid valve 11 allows compressed breathing gas toenter compartment A through opening 25. In a second position P2 thesolenoid valve 11 allows to vent compressed air in compartment A throughventing tube 22. The solenoid valve 11 is only shown schematically inthe Figures; its design allows switching between positions P1 and P2without passing an intermediate position in which the lumina of tubes 10and 21 and the compartment A are in simultaneous communication. Thesolenoid valve is actuated by a solenoid valve control unit 12 receivingactuation signals from the microprocessor unit 13 via line 20. Themicroprocessor unit 13 and the solenoid valve control unit 12 areenergized by a dry battery (not shown). The three-way solenoid valve 11of the embodiment of FIGS. 1 and 2 a to 2 h can be exchanged for a pairof solenoid valves 11′, 11″ actuated by a solenoid valve control unit12′ (FIG. 1c ). Reference numbers 4′, 10′, 21′ and 25′ identify elementscorresponding to elements 4, 10, 21 and 25, respectively, of theembodiment of FIGS. 1 to 2 h.

After leaving the gas cylinder the compressed breathing gas isdecompressed in controlled manner (not shown) to a working pressure,which is kept about constant during CPR. The gas of working pressure issuitable held in a reservoir from which the gas of working pressure isadduced to the compartment A via tube 10 so as to provide it at an aboutconstant gas pressure over time. This allows the provision of acontrolled compression force via the piston 5, the plunger 6, and thepad 7 to the chest of a patient. Since the adduction of compressed gasthrough tube 10 and the build-up of gas pressure in compartment A is adynamic process governed by the pressure of the gas in the gasreservoir, the gas pressure in the compartment 10 (the pressure of the“provided” driving gas) is not in equilibrium with the pressure of thedriving gas at the source over an initial portion of the compressionphase.

The compression pad 7 is loosely placed on the chest 30 of a person tobe provided chest compressions (FIG. 1). The person is in a recumbentposition with the pad 7 placed on the skin 31 above the sternum 32.Reference numbers denote: 33, right ventricle; 34, left ventricle; 35,esophagus; 36, descending aorta; 37, body of the eight thoracic vertebra(T8); 38, spinal cord; 39 left arc of ribs.

The function of the apparatus of the invention will now be explainedwith reference to FIGS. 2a through 2 h.

The position at the start of dispensation of chest compression is shownin FIG. 2a , which corresponds to FIG. 1 except for that only the skin31 of the patient's chest in the sternal region is shown. In the Figuresthe uncompressed level of the skin 31 at the application site of the pad7 is designated O. The solenoid valve 11 is in the venting position P1and the plunger is in an unloaded state. By opening of the venting valve11 (valve position P2) compressed air is made to flow from the gascylinder to tube 10 and to enter compartment A through opening 25. Theincreasing air pressure in compartment A starts to force the piston 5downwards in the direction of bottom wall 3 (FIG. 2b ; start of downwardmovement of piston 5 indicated by an arrow). At start the south pole Sof the magnet 14 is disposed between Hall switches 15 and 16. During itsdownward movement (FIG. 2c ; valve 11 in position P2; skin level atintermediate position P during downward movement) the south pole S ofthe magnet 14 passes Hall switches 16, 17 and stops at the level ofswitch 18 (skin level R at initial full compression=Compression Depth inan initial cycle of CPR). It stops at because, at this level, thecompression force of the compressed gas acting on the piston 5transferred by the pad 7 to the patient is balanced by the resilientcounterforce of the compressed chest tissues. As explained above, themicroprocessor unit 13 keeps track of the position of the piston 5during its downward (and upward, if desired) movement, and recognizesthe exact moment at which the piston 5/plunger 6/rod 7 assembly hasreached its extreme position during its downward movement (FIG. 2d ,indicating the infinitesimal last downward movement of piston 5/plunger6/rod 7 assembly prior to stop with the valve 11 in position P1; FIG. 2e, the moment of stop with the valve 11 in position P1; and, at the samemoment, FIG. 2f , an immediate switch of the valve 13 from position P1to P2. As explained above and if desired, the switch of the solenoidvalve 11 from P1 to P2 can be made to occur slightly earlier, that is,prior to the piston 5/plunger 6/rod 7 assembly reaching its downwardstop position by programming of the microprocessor 13 correspondingly.The recognition of the time when the piston 5/plunger 6/rod 7 assemblyreaches its lower end or bottom position moment thus is used for controlof the solenoid valve so as to switch it from position P2 to P1. Theflow of compressed gas into compartment A is stopped at the moment wherethe piston 5/plunger 6/rod 7 assembly reaches the desired extremeposition (Compression Depth) or slightly before that moment. Thereby theprovision of driving gas is optimized and thus economized. This is ofparticular importance for a CPR apparatus to be used outside facilitieslike a hospital where practically unlimited resources of compressed gasof various kind are available. In the state of the apparatus shown inFIG. 2f compartment A is vented via tube 21. If the driving gas is abreathing gas the vented gas or a portion thereof, which is still of aslightly higher pressure than ambient air, can be adduced to thepatient's lungs via a breathing mask or by intubation (not shown). Theventing of compartment A stops the load on the piston 5/plunger 6/rod 7assembly (FIG. 2g ) and thus the compression of the patient's chest. Theresilient nature of the chest makes it expand and push the piston5/plunger 6/rod 7 assembly back to its start position (FIG. 2h ).

The microprocessor unit 13 of the apparatus of the invention isprogrammed in a manner so as to sample and store positional data overone or several cycles, and to use such data for control of a latercycle.

For reasons of simplicity and to better illustrate the principles of theinvention the apparatus the invention shown in FIGS. 1 to 2 h has beensimplified in respect to commercially available apparatus of this kindin regard of ancillary features. Thus, the upward movement of the piston5/plunger 6/rod 7 assembly of this embodiment of the apparatus of theinvention is passive, that is, driven by the resilient force of thepatient's chest, whereas it can be advantageously be driven by means ofthe compressed breathing gas used in the apparatus. In such case asubstantially more complex arrangement of valves and gas lines foradducing compressed gas to compartment B and venting it from there isrequired. The provision of and additional pressure means for actively,that is, substantially independent of the resilient forces of acompressed chest, returning the piston to its start position doeshowever not change the principles of the present invention, which evenmight be used to optimize the use of compressed gas in the displacementof the piston 5/plunger 6/rod 7 assembly in such upward (decompressing)direction.

In a second embodiment of the CPR apparatus of the invention 101 shownin FIGS. 4a to 4d the position of the piston 105 and thus thecompression depth is determined by means of a source of visible light114 and a number of photo detectors 143, 144, 145, 146, 147, 148, 149,all disposed in a radial direction, with the light source 114 innermost,on the inner face of bottom wall 103. The light source 114 is a redlight photodiode whereas the photo detectors 113-119 are silicon basedphotodiodes operated in photoconductive mode.

The narrow and substantially parallel beam of light 124′ of thephotodiode 114 is directed at the lower face of the piston 114, which isprovided with a ring mirror 130, at an angle .alpha. and in the same aradial direction in respect of the piston 105 axis as that of thedisposition of photo detectors 113-119. The incident beam 124′ isreflected at the same angle .alpha. in the direction of photo detectors113-119 disposed on the bottom 103. The distance between the inner faceof the bottom 103 and the lower face of the piston 105 provided with themirror element 130 determines which of the photo detectors 143-149 ishit by the reflected beam 124″. In a position of the piston 105 near thebottom wall 103 (distance d1, FIG. 4c ) the reflected beam 124″ hits thenext but innermost photo detector 148, whereas in a position of thepiston near the top wall 104 (distance d2, FIG. 4d ) the next butoutermost photo detector 144 is hit. During a downward movement of thepiston 105 the reflected beam 124″ thus will sweep, depending on itsstart position and its end position (Compression Depth position) overall or only some of the photo detectors in a radially inward direction.The photo diode 114 and the photo detectors 143, 144, 145, 146, 147,148, 149 are connected to a microprocessor unit 113 via separateconductors 114′, 143′, 144′, 145′, 146′, 147′, 148′, 149′, respectively,which are bundled in a cable 131. The microprocessor 113 uses thesignals from the photo detectors 143-149 in a time frame to control gasflow in the apparatus 100 by a solenoid valve 111 operated by a solenoidcontrol unit 112 in a manner corresponding to that described in Example1 for the electric signals generated by the Hall-effect switches 15, 16,17, 18, 19. In FIGS. 4a-4d reference numbers 106, 110, 120, 121 refer toa plunger 106 carrying a chest compression pad (not shown), a tube 110for adducing compressed breathing gas, to a electrical connection 120between the microprocessor unit 112 and the solenoid control unit 112,and to a tube 121 for venting compressed breathing gas used fordisplacement of the piston 105, respectively.

In a third embodiment of the invention shown in FIG. 5 the stroke of thepiston 305 is limited by an annular stop 320. At its lower extremeposition the piston 305 hits and thereby closes a contact switch 315 ofan electrical circuit comprised by a microprocessor unit 313. Themicroprocessor unit 313 thereby receives information about the moment atwhich the Compression Depth is reached. Based on this information themicroprocessor's 313 issues a closing command to the control unit 312 ofsolenoid valve 311, in particular in a following cycle prior to theexpected time of contact. In this embodiment the provision of a gasinlet tube 340 to provide driving gas to the closed lower chamber B ofthe cylinder housing 302, 303, 304 illustrates the principle of assistedpiston 305 return that can be applied to all embodiments of theinvention, if desired.

Example 2

Solenoid Valve Control Program

In the following an example of a simple main valve control program isprovided (Table 1). In the example consideration is given to one Halleffect element (Hall switch), which is placed at about a desired levelof piston 5/plunger 6/rod 7 assembly stop (bottom level). Time open forthe decompression main valve is set to 300 ms; while this parameter isfixed in the Example, it could be controlled in precisely the same wayas time open for the compression main valve.

TABLE 1 Initialize: set t_open = 300 [ms] set adjust = true (Parallelprocess #1, controls main valves) While true do is_down = falsemain_valve_comp = true /opens compression main valve wait    t_open/holds main valve open for t_open ms main_valve_comp = false /closescompression main valve wait  300 − t_open /wait the rest of thecompression phase main_valve_decomp = true /opens decompression mainvalve wait    300 /waits until whole cycle is complete main_valve_decomp= false /closes decompression main valve if adjust = true if is_down =true t_open = t_open − 20 /decreases t_open else t_open = t_open + 10/increases t_open adjust = false /adjustment is now complete end if endif end while (Parallel process #2, samples hall_element_signal input andupdates variable “is_down”) while adjust = true do hall_effect_sample =read_digital_input_signal_of_hall_effect_element is_down = is_down orhall_effect_sample /true if piston has reached end while /hall elementduring cycle

When the program starts the program variable (t_open), which controlsthe time the air supply port for the compression phase is open, is setto 300 ms, which is the maximum possible value. The apparatus thenperforms one cycle (compression and decompression) with this setting.During the cycle the signal from the Hall effect element is sampled. Ifthe piston reaches the bottom of the cylinder it will be registered bythe Hall effect sensor signal as a high voltage, sampled by the function“read digital input signal_of_hall_effect_element” and then written tothe variable is_down. is_down is the variable that indicates whether thepiston has reached its bottom position during the cycle, and thendetermines which adjustment of t_open shall be performed. If a triggerwas detected (and is_down set to true), than the variable t_open islowered by 20 ms. This is repeated for every cycle until there is notrigger detected. During this last cycle the piston is likely to havestopped just before it reached the Hall effect element, such as a fewmillimeters from demand position. As is_down now is false the variablet_open is increased by 10 ms, which makes the piston move a little bitfurther down next cycle; this is then considered to be the finalposition at which the update procedure stops (since the variable adjustis set to false it cannot become true again). This setting will be usedfor the rest of the treatment or may be changed after some time such as,for instance, 10 minutes from start, to adapt the compression to theaforementioned change in physical properties of the chest. A blockdiagram of the program is shown in FIG. 3.

Example 3

The effect of the method of the invention in the control of compresseddriving gas is demonstrated by three experiments illustrated in FIGS.5a-5c . The experiments were carried out with an air-drivenreciprocating CPR device mounted on a test bench. The CPR apparatuscomprises a compression cylinder 208 comprising an upper compartment 219and a lower compartment 220 delimited in respect of each other by apiston 216 arranged displaceably in the cylinder 208. The apparatusfurther comprises a breast compression pad 210 attached to the piston216 via a shaft 211, a valve control unit 212 with a valve manifold, anda gas line 213 supplying driving gas from source of compressed gas (notshown) to the compression cylinder via the valve control unit 212. Thestroke (str) of the piston 216 is limited to 55 mm by means of upper 217and lower 218 stroke limiters disposed in the upper and lowercompartments, 219, 220, respectively. The gas pressure in the uppercompartment 219 is measured by a manometer 214. The test bench comprisesa flat base 201 on which the CPR apparatus is mounted via a pair of legs209. The compression pad 210 abuts a top face of a sternal plate 204resting on a support 202 via an interposed force sensor 203. The support202 rests displaceable in a vertical direction on the base 201 viacompression coil means 205, which mimic a resilient chest. A linearsliding rail 206 fixed at the base 201 allows to read the position ofthe sternal plate 204 and the compression pad 210 by means of a linearslide guide 207 running on the rail 206. The slide guide 207 comprises aposition sensor. As indicated by reference numbers 203′, 207′, 212′ and214′ signals from the force sensor 203, the position sensor 207, thevalve control unit 212 and the manometer 214 are electricallytransferred to a control unit 215 in which they are stored and fromwhich the can be recalled and displayed. The resisting force of thecompression coil means 205 against further compression at a CompressionDepth of about 50 mm was set to about 500 N (FIG. 5 a, 477 N) or half ofthat value, about 250 N (FIG. 5 b, 239 N; FIG. 5 c, 230 N). These andother parameter values are listed in Table 2. In all experiments thepressure of the driving gas fed to the compression cylinder was 2.91bar.

TABLE 2 Recip- Max. Inlet rocating cylinder Compr. Decompr. valve Compr.Exp. frequ., pressure, Force phase, phase, open, depth, # 1/min bar (N)sec sec sec mm a 98 2.91 477 0.31 0.30 0.31 52.1 b 99 2.88 239 0.31 0.300.31 56.0 c 99 1.36 230 0.30 0.31 0.09 55.5

Experiment (a), FIG. 5a , reflects the situation at the start of CPR,that is, during the first compressions administered to a patient. Toprovide optimal treatment it is necessary to obtain full compression,that is, a Compression Depth of about 50 mm for the average adultperson, right from start and at an adequate rate of about 100compressions per minute and even more. For this reason the minimumpressure of the driving gas is set to about 3 bar or more. This sufficesto develop a compression force of about 500 (477 N in the experiment),by which the compression coil means 205 are compressed to a full stroke(str, FIG. 6), the lower stroke limiter 218 being reached at point L inFIG. 5 a.

Experiment (b), FIG. 5b , reflects the situation after provision ofcompressions to a patient for a few minutes. During this time period theresistance of the chest diminishes by about 50%. The force ofcompression necessary to obtain a desired Compression Depth of about 50mm thus is substantially reduced. In this experiment the resistance ofthe compression coil means 205 is set to about half (239 N) of theresistance in experiment (a). A compression profile similar to that ofexperiment (a) is obtained, except for the downward stroke occurringconsiderably faster, the lower stroke limiter 218 being reached at M. Inboth experiments (a) and (b) the driving gas inlet valve is open duringthe entire compression phase. It is closed simultaneously with theopening of the venting valve, by which the pressure in the compressionchamber is released to allow the piston 216 and the compression pad 204return to their starting position defined by upper stroke limiter 217.This return movement is supported by means of driving gas being fed to alower chamber 219 in the housing (FIG. 6).

In experiment (c), FIG. 5c , substantially the same time v, compressionpad displacement curve is obtained as in experiment (b). Experiment (c)differs from experiment (b) only in that the valve by which driving gasis adduced to the upper chamber 219 is kept open for a comparativelyshort time only. It is made to close at N (FIG. 6) even before theCompression Depth is reached. The final or maximum gas pressure in thecompression compartment is thereby limited to about half of the pressurein experiment (b), and a corresponding saving of driving gas isobtained.

Experiments (a) to (c) demonstrate that up to 60 percent and even up toabout 70 percent of driving gas can be saved by the method and theapparatus of the invention. This has been confirmed in in-vivoexperiments in a pig model.

What is claimed is:
 1. A method of sensing a compression depth in areciprocating apparatus for cardio-pulmonary resuscitation (CPR) havinga reciprocating plunger assembly within a non-reciprocating housing, themethod comprising: sensing, with an array of magnetic sensors on thehousing and for different positions of the plunger assembly, a magneticfield emanating from a permanent rod magnet reciprocating with theplunger assembly, the permanent rod magnet having a pole facing thehousing, the array of magnetic sensors including more than two magneticsensors positioned between an upper extent of a stroke of the plungerassembly and a lower extent of the stroke, and the plunger assemblyincluding a counterweight diametrically opposite to the rod magnet;creating an electrical signal in at least one of the magnetic sensors bysweeping the at least one of the magnetic sensors with the magneticfield; and transmitting the signal to a microprocessor unit that recordsan arrival time of the signal.
 2. The method of claim 1, furthercomprising driving the plunger assembly by a compressed gas.
 3. Themethod of claim 2, further comprising utilizing the signal, by themicroprocessor unit, to control the compressed gas driving the plungerassembly.
 4. The method of claim 1, further comprising utilizing thesignal, by the microprocessor unit, to control reciprocation of theplunger assembly.
 5. An apparatus for cardio-pulmonary resuscitation(CPR) comprising: a cylinder housing having a diamagnetic wall; adiamagnetic plunger assembly configured to reciprocate within thehousing; a rod magnet configured to emit a magnetic field and configuredto reciprocate with the plunger assembly, the rod magnet having a polefacing the housing, and the plunger assembly including a counterweightdiametrically opposite to the rod magnet; and an array of magneticsensors on a surface of the housing, the array of magnetic sensorsincluding more than two magnetic sensors positioned between an upperlimit of a stroke of the plunger assembly and a lower limit of thestroke, each of the sensors in the array of magnetic sensors beingconfigured to detect when the magnetic field exceeds a threshold duringeach of a plurality of reciprocation cycles and, when the magnetic fieldexceeds the threshold, to transmit a signal.
 6. The apparatus of claim5, further comprising a microprocessor connected to the array ofmagnetic sensors and having a memory, the microprocessor beingconfigured to receive the signals and to store the received signals inthe memory.
 7. The apparatus of claim 6, in which the array of magneticsensors is further configured to sense a current position of the plungerassembly.
 8. The apparatus of claim 7, further comprising a valveconfigured to control a volume of pressurized gas entering the housing,the valve further being configured to be controlled by themicroprocessor based on the stored signals and the current position ofthe plunger assembly.
 9. The apparatus of claim 8, in which the housingcomprises a first chamber at a first side of the housing and a secondchamber at a second side of the housing that is opposite the first side.10. The apparatus of claim 9, further comprising a second valveconfigured to control a second volume of pressurized gas leaving thesecond chamber of the housing, the second valve further being configuredto be controlled by the microprocessor based on the stored signals andthe current position of the plunger assembly.
 11. The apparatus of claim5, in which the plunger assembly further comprises diametricallyopposite, axially extending slits in a sidewall of the plunger assembly,and in which the housing further comprises diametrically opposite,axially extending flanges protruding from an inner face of a sidewall ofthe housing and configured to mate with the axially extending slits ofthe plunger assembly.